docs: def segment_area(seg):
      '''Compute the surface area of a segment.
  
      Approximated as a conical frustum. Does not include the surface area
      of the bounding circles.
      '''
      r0 = seg[0][COLS.R]
      r1 = seg[1][COLS.R]
      h2 = point_dist2(seg[0], seg[1])
      return math.pi * (r0 + r1) * math.sqrt((r0 - r1) ** 2 + h2)